Hunter Industries provides a Handbook of Technical Irrigation Information which includes formulas that are extrememly helpful in determining efficient watering schedules. This article is based on the information provided in the Handbook.
The Run Time Formula allows you to calculate how long you need to run your sprinkler system to compensate for the water lost in evapotranspiration. The formula is designed to calculate for specific crops watered with a system running at specific precipitation rates.
T = Run time in minutes
Eto = Evapotransporation rate, in inches
Kc = Crop coefficient, percent
Pr = Precipitation rate of the area, in inches per hour
Ea = Application efficiency of the sprinkler system, percent
60 =Constant for conversion of area, flow, inches per hour, and inches per day into common units.
If you wanted to determine the total monthy run time for a soccer field with a total monthly ETo of 2.0 inches, a crop coefficient for turf during the warm season of 0.70, and sprinkler precipitation rate of 0.50 inches per hour with an application efficiency of 75%, you would apply the formula as follows:
T = 224 minutes of total monthly watering time
Thus if the system is programmed to water on 10 days during a month, the zone watering time would be 22.4 minutes (1/10th) of the total monthly requirement.
The Evapotranspiration rate, or ETo, refers to the rate at which plants lose water to evaporation. Humidity, temperature, and other factors are taken into account to determine the Eto rate at a particular time. Seasonal changes, such as humidity and temperature, cause ETo rates to change. Daily, and even hourly fluctuations in temperature and humidity will cause changes, but for most basic irrigation purposes, such as lawns and gardens, average seasonal ETo rates are sufficient for efficiently scheduling watering run times.
ETo rates for your area are provided by many reliable sources, such as the National Climatic Center. the following chart was provided by the NCC in Sheville, North Carolina, and reports the total monthly ETo rates for the Austin area.
Crop coefficient (abbreviated Kc) refers to ETo water loss for specific plants. This allows you to adjust for ETo variations between soft leafy plants, like ferns, which lose water faster than hard, waxy leafed plants, such as groundcovers. Warm season grass species, such a Bermuda, lose water differently than cool season grass species like rye.
Application efficiency, or EA, refers to how evenly your sprinkler system is watering- the uniformity of coverage. The only really accurate way of measuring EA is to run a sprinkler system audit. dThis can be done using catch cans, a rain gauge, or any non-tapered cup or can. See the links listed at the bottom of this article for a detailed article on performing an audit.
Other important factors are soil consistency and active root zone depth. All of these need to be considered when calculating a water efficient schedule for your sprinkler system.